Refractive Index Changes in Optical Glass occasioned by Chilling and Tempering

JSGT 1923 V07 T199-T207

 It has long been known that the two processes of chilling and tempering produce important changes in the structure and properties of metals, the best known instance being that of the hardening and tempering of steel. It is therefore to be expected, as glass has elastic properties analogous to those of metals, that similar heat treatment will in this case also produce changes which might well be indicated by a change of refractive index. Indeed for many years it has been known that fine-annealed glass may have a refractive index different from that of the same glass before fine annealing, a fact which suggests that the chilling of glass might produce peculiar properties. The well-known strengthening of glass objects produced by chilling them superficially while within or above the annealing range might also be a case in point, although a plausible explanation is afforded by the state of compression known to be occasioned in the outer layer by such a process.

F. Twyman & F. Simeon